Provide top-level documentation for memory management facilities that
was marked as TODO in refptr & co matches. See e.g. the following
commits for context:

- e82b4fab (libgolang: Objects refcounting (initial draft))
- b2253abf (libgolang: Rename refobj -> object)
- fd2a6fab (libgolang: Fix globals atexit race condition of ~refptr vs
  access from another thread)